It has previously been proposed to make liquid-to-liquid pressure transducers, but the successful manufacture of an accurate and reliable pressure transducer of this type has presented serious problems. In the case of capacitive pressure transducers for measuring the pressure of gases, reliable transducers have been developed, and one such transducer is shown, for example, in U.S. Pat. No. 4,207,604, granted June 10, 1980, to the assignee of the present invention. However, when it is desired to measure the pressure between two liquids, certain problems arise. Initially, with the plates of the pressure transducer being spaced quite close together, in the order of 1/2 of a thousandth of an inch up to several thousandths of an inch, the fluid of which the pressure is to be measured, normally will not easily flow into and completely fill the space between the two plates. Of course, trapped air bubbles will create errors, when the trapped air expands or contracts with changes in temperature. In addition, if a separate reference capacitor is employed, the significant changes in capacitance of the transducer itself may make it difficult to match the capacitance of the transducer with that of the reference capacitor, particularly under variable conditions such as changing temperatures or the like. Further, the ratio of the variable to the fixed capacitor may change significantly when the transducer is liquid-filled, as compared with a gas filled transducer, and this may create problems in the calibration of the device.
Accordingly, a principal object of the present invention is to provide an inexpensive and reliable liquid-to-liquid capacitive pressure transducer.